1. Field of the Invention
This invention relates to artificial joints and in particular to artificial joints of the ball and socket type.
2. Description of the Prior Art
As is well known in the art, artificial hip and shoulder joints conventionally employ ball and socket articulations. The socket portion of the joint is attached to one bony structure, for example, the pelvis for a hip reconstruction. The ball is connected to an arm composed of a neck and a stem or shaft, and the stem or shaft is embedded in another bony structure, for example, the femur for a hip reconstruction.
A number of methods are known for retaining the ball in the socket. In the most common method, referred to herein as the "semi-constrained" construction, the patient's own anatomy, i.e., his muscles, tendons and ligaments, are used to retain the ball within the socket. For this construction, a hemispherical socket typically is used which allows the ball and its attached arm the maximum amount of movement without contact of the arm with the edge of the socket. The surgeon, when installing such a semi-constrained joint, aligns the ball and socket as closely as possible with the patient's natural anatomy so that the patient's movements do not tend to dislocate the ball from the joint.
In order to increase the inherent stability against dislocation of semi-constrained constructions, it has become conventional to add a cylindrical portion to the hemispherical socket to make it deeper. Although the ball is not physically constrained by the socket by this adjustment, the ball does have further to travel than if just a hemisphere had been used and thus some reduction in the propensity towards dislocation is achieved.
A recent study by the Mayo Clinic, which appeared in the December, 1982 edition of The Journal of Bone and Joint Surgery, reported a dislocation frequency of 3.2% for 10,500 hip joint implant procedures using the semi-constrained construction. Such dislocations essentially make the patient immobile and can necessitate a second operation. Because the success of semi-constrained constructions depends upon achieving a relatively precise alignment between the patient's anatomy and the components of the artificial joint and because a first operation and the healing process thereafter usually destroy or distort anatomical landmarks, even higher dislocation frequencies are encountered for second and subsequent implantations.
An alternative to the semi-constrained construction is the "constrained" construction wherein the ball is physically constrained within the socket. The present invention is directed to this type of construction.
In the constrained construction, a spherically-shaped bearing surrounds the ball and serves as the socket. The bearing is attached to a fixation element which is embedded in, for example, the patient's pelvic bone. The bearing encompasses more than one-half of the ball and thus constrains the ball and its attached arm from dislocation.
The bearing is made of metal or, more typically, of plastic, such as ultra-high molecular weight polyethylene (UHMWPE).
An example of a constrained construction using a metal socket bearing is shown in Noiles, U.S. Pat. No. Re. 28,895 (reissue of U.S. Pat. No. 3,848,272). In a practical sense, this joint can be said to be non-dislocatable. The force required to extract the metal sphere from the enclosing metal socket bearing is more than several thousand pounds. Accordingly, in use, rather than the metal ball dislocating from the metal socket bearing, any overly severe dislocating leverage will cause the fixation element to be disrupted from the bone in which it has been embedded.
Notwithstanding the fact that metal balls in metal socket bearings are non-dislocatable, they are used in only a minority of joint reconstructions because the medical profession is not in agreement that a metal sphere in a metal bearing is as biologically acceptable as a metal sphere in a UHMWPE plastic bearing, even though clinical use over 15 years has failed to show the metal to metal joint to be inferior to a metal to plastic joint.
In view of this prejudice against metal socket bearings, the constrained constructions in most common use today employ plastic bearings. For these constructions, a pre-assembled ball and socket bearing assembly is supplied to the surgeon. The manufacturer constructs the assembly by forcing the bearing over the ball. The more of the ball which is encompassed by the bearing, the greater the required assembly force, and the greater the constraining force to prevent post-operative dislocation of the joint. To aid in assembly, the socket bearings are usually heated to a non-destructive temperature (for example 70.degree.-80.degree. C. for UHMWPE). Plastic in general, and UHMWPE in particular, has a large coefficient of thermal expansion and such thermal expansion due to heating significantly aids in assembly.
An example of a constrained artificial joint employing a plastic bearing is shown in Noiles, U.S. Pat. No. 3,996,625. As can be seen in FIGURE 1 of this patent, a plastic bearing 17 fitted with a metal reinforcing band (un-numbered) extends beyond the diameter of ball 24 so as to physically constrain the ball within the bearing. The bearing itself is attached to fixation element 12. The metal reinforcing band is assembled over the lip of the opening of bearing 17 after that bearing has been forced over sphere 24. The reinforcing band increases the force required to dislocate the joint.
In practice, the design shown in FIGURE 1 of U.S. Pat. No. 3,996,625 has been found to resist direct dislocating forces of approximately five hundred pounds. Moreover, this joint has been found to suffer post-operative dislocations in fewer than 0.5% of the implantations performed. This is significantly better than the 3.2% dislocation frequency reported for semi-constrained constructions in the Mayo Clinic study discussed above.
Although highly successful, constrained ball and socket joints employing plastic socket bearings, including joints of the type shown in U.S. Pat. No. 3,996,625, have suffered from the disadvantage that unless they were designed to provide only a small constraining force, i.e., if they were designed to encompass only slightly more than half the ball, they could not conveniently be assembled in the operating room. Specifically, the force required to assemble the bearing onto the ball, even for a heated bearing, has in general been too great to be conveniently applied by a surgeon in the midst of a surgical procedure. Although mechanized assembly devices capable of generating the necessary force could have been used by surgeons, in practice, surgeons have preferred to use constrained joints where the socket bearing and ball have been pre-assembled.
The lack of a truely convenient way to assemble constrained joints in the operating room has limited the usefulness of these joints in the following ways. First, at the time of an initial joint implantation, the need to pre-assemble the socket bearing and the ball portion of the joint has meant that the surgeon has had to make the decision to employ a constrained joint prior to implantation of the ball's fixation element, e.g., prior to implantation of the stem or shaft portion of the arm attached to the ball. He could not postpone the decision as to the type of joint to use until after implantation of the fixation elements for both the ball and socket portions of the joint, when he might have had a fuller appreciation of the patient's medical condition and anatomy.
Similarly, if after an initial implantation, a patient proved to be particularly prone to dislocations with a semi-constrained construction, there has been no truly convenient way to change to a constrained construction employing a high-constraint plastic socket bearing without removing the ball portion of the original joint from the bone in which it had been originally implanted. Along these same lines, there has been no convenient way to replace a worn socket bearing of the constrained type without completely removing the entire ball portion of the joint from the patient.